Reuven is planning a trip to Israel. Shimon, knowing this, gives Reuven money for tzedakah with the expectation that Reuven will distribute it when he gets to Israel. Reuven and Shimon both regularly support Israeli institutions.
Why do we do this? Is there some extra benefit to Reuven because he is participating in Shimon's mitzvah? Does Shimon benefit because Reuven might choose a recipient that Shimon hasn't previously supported? Is there particular value to cash donations (versus the checks Reuven and Shimon use for their other tzedakah)?